This invention relates, generally, to induction machines and, more particularly, to detection of asymmetry of a rotor in an induction machine.
Induction machines are valued in commerce and industry because they are relatively simple to build and generally robust. Examples of induction machines include induction motors and induction generators.
In a typical induction motor, a rotor is magnetically coupled with a stator which has windings that conduct alternating current (xe2x80x9cACxe2x80x9d). The rotor includes conductive bars which are electrically shorted together by end rings in a configuration commonly referred to as a xe2x80x9csquirrel cage.xe2x80x9d The conductive bars are mounted within magnetically conductive material. Also, the rotor is coupled to a shaft which provides torque as output to a load connected to the shaft.
Typically, three phase AC current is applied to the windings of the stator. The AC current applied to the windings of the stator induces current in the conductive bars of the rotor. The current induced in the conductive bars of the rotor interacts with a magnetic flux induced by the AC current applied to the windings of the stator to produce torque for a load connected to the shaft and cause the shaft to turn.
An asymmetry in the conductive bars or the end rings generally adversely affects performance of the induction motor by creating a locally increased electrical resistance. For example, a crack or porosity in a conductive bar or an end ring usually leads to undesirable results such as vibration, heating, or torque pulsation.
As described in Sims, U.S. Pat. No. 3,875,511, an early technique for detecting rotor asymmetry faults was to test the rotor prior to assembling the rotor into a stator. To use such a technique to test a motor that has been operational requires disassembly of the rotor from the induction motor to test the rotor in isolation from the stator. A common drawback of disassembly includes time spent in disassembling the rotor from the induction motor for testing and reassembling the rotor in the induction motor following the testing.
As described in Kliman et al., U.S. Pat. No. 4,761,703, a subsequent technique for detecting a rotor asymmetry or fault required the shaft to be connected to a load which served to provide slip between the rotor and the stator field for measurement of current induced in the conductive bars by usual operation of the induction motor. Such a technique commonly presents a difficulty of requiring mechanical alignment between the shaft and the load. So, the technique typically entails the task of mechanically connecting a load in order to test the induction motor before use. Delaying testing for asymmetry until the induction motor is installed with a final load often causes additional difficulties of downtime or inefficiency.
Accordingly, a need exists for a test for an asymmetry in an induction machine while the induction machine is assembled and without requiring connection of a mechanical load. A further need exists for the test to increase convenience, simplicity, and accuracy of asymmetry detection.
An induction machine asymmetry detection instrument includes an interconnector and a voltmeter. The interconnector is configured to connect an electric source across a first terminal and a second terminal of a stator of an induction machine. The induction machine includes a rotor disposed for magnetic coupling with the stator. The interconnector is further configured to cause a flow of direct current between the first terminal and the second terminal of the stator during a rotation of the rotor when the induction machine is substantially unloaded. The voltmeter is connectable across the second terminal and a third terminal of the stator. A detection by the voltmeter of a meaningful voltage across the second terminal and the third terminal of the stator concurrent with the flow of direct current between the first terminal and the second terminal and concurrent with the rotation of the rotor serves to indicate an asymmetry of a portion of the induction machine.
An induction machine asymmetry detection method includes controlling a flow of direct current between a first terminal and a second terminal of a stator of an induction machine during a rotation of a rotor of the induction machine when the induction machine is substantially unloaded. A voltage is measured across the second terminal and a third terminal of the stator concurrent with the flow of direct current between the first terminal and the second terminal and concurrent with the rotation of the rotor to determine the presence of an asymmetry of a portion of the induction machine.